Carefully flip the board over and use tweezers to remove the blue-black ribbon located underneath.

If your ABXY buttons aren't working, you can peel off the clear sticker with the metal contacts to clean off the corrosion that may build up underneath, as shown in this video.

Use a sharp object such as an X-ACTO knife to scrape off the corrosion, as shown; a similar procedure can be followed for a malfunctioning directional pad. The device can then be reassembled, following these steps in reverse order.